Ion mobility spectrometer instrument and method of operating same

ABSTRACT

An ion mobility spectrometer instrument has a drift tube that is partitioned into a plurality of cascaded drift tube segments. A number of electric field activation sources may each be coupled to one or more of the plurality of drift tube segments. A control circuit is configured to control operation of the number of electric field activation sources in a manner that applies switched electric fields at a specified switching rate to the drift tube segments to thereby produce at the ion outlet only ions having a predefined ion mobility or range of ion mobilities.

CROSS-REFERENCE TO RELATED APPLICATION

This patent application claims the benefit of, and priority to, U.S. Provisional Patent Application Ser. No. 61/021,785, filed Jan. 17, 2008, the disclosure of which is incorporated herein by reference.

GOVERNMENT RIGHTS

This invention was funded in part by grants from National Institute of Health, NIH (Grant Nos. Ag-024547-01 and P41-RR018942); the United States Government may have rights in this invention.

FIELD OF THE INVENTION

The present invention relates generally to ion separation instruments, and more specifically to instruments that operate to separate ions in time as a function of ion mobility.

BACKGROUND

Ion mobility spectrometers are analytical instruments that are used to separate ions in time as a function of ion mobility. It is desirable to be able to control electric fields applied to such instruments in order to investigate properties of charged particles.

SUMMARY

The present invention may comprise one or more of the features recited in the attached claims, and/or one or more of the following features and combinations thereof. An ion mobility spectrometer instrument may comprise a drift tube partitioned into a plurality of cascaded drift tube segments each defining an ion inlet at one end and an ion outlet at an opposite end. The plurality of cascaded drift tube segments may an ion outlet at an opposite end. The plurality of cascaded drift tube segments may further define an ion elimination region between an ion outlet of each drift tube segment and an ion inlet of an adjacent drift tube segment. An ion source may be coupled to the ion inlet of a first one of the plurality of cascaded drift tube segments. A number, M, of electric field activation sources may each be operatively connected to one or more of the plurality of drift tube segments such that, when activated, each establishes a repulsive electric field in a different one of the first M ion elimination regions and in every following Mth ion elimination region, and also establishes an electric drift field in all remaining ion elimination regions and in all of the plurality of cascaded drift tube segments. A control circuit may be configured to sequentially activate each of the number, M, of electric field activation sources for a time duration while deactivating the remaining number, M, of electric field activation sources a number of times to thereby cause only ions generated by the ion source that have a predefined ion mobility or range of ion mobilities to traverse the drift tube.

The ion inlet of the first one of the plurality of cascaded drift tube segments may define an ion inlet of the drift tube and the ion outlet of the last one of the plurality of cascaded drift tube segments may define an ion outlet of the drift tube. Ions may travel from the ion inlet of the drift tube through the ion outlet of the drift tube under the influence of the electric drift fields established by the number, M, of electric field activation sources. Ions that do not have the predefined mobility or range of ion mobilities may be filtered away by the repulsive electric fields established by the number, M, of electric field activation sources.

The ion source may be configured to continuously generate ions. The continuously generated ions may enter the ion inlet of the drift tube under the influence of the electric drift field sequentially established by each of the plurality, M, of electric field activation sources in the first drift tube segment. Alternatively or additionally, the ion inlet of the first drift tube segment may comprise an ion gate. The control circuit may be configured in this embodiment to control the ion gate to selectively allow entrance of discrete packets of ions generated by the ion source into the ion inlet of the drift tube. Alternatively or additionally still, the ion source may comprise at least one ion separation instrument configured to separate ions in time as a function of one or more molecular characteristics. The at least one ion separation instrument may include at least one of a liquid chromatograph, a gas chromatograph, an ion mobility spectrometer, a mass spectrometer, and a capillary electrophoresis instrument.

Each of the number, M, of electric field activation sources may be programmable to establish, when triggered, the electric drift and repulsive fields for the time duration. The control circuit may be configured to sequentially activate the number, M, of electric field activation sources for the time duration by sequentially triggering the number, M, of electric field activation sources. Alternatively, the control circuit may be configured to sequentially activate the number, M, of electric field activation sources for the time duration by triggering one of the number, M, of electric field activations sources with remaining ones of the number, M, of electric field activation sources being triggered by operation of a previously triggered one of the number, M, of electric field activation sources.

The time duration may define the mobility or range of mobilities of ions that will traverse the drift tube.

Each of the number, M, of electric field activation sources may be configured, when activated, to establish the electric drift and repulsive fields in the form of constant electric fields for the time duration. Alternatively, each of the number, M, of electric field activation sources may be configured, when activated, to establish the electric drift and repulsive fields in the form of linearly varying electric fields for the time duration. Alternatively still, each of the number, M, of electric field activation sources may be configured, when activated, to establish the electric drift and repulsive fields in the form of non-linearly varying electric fields for the time duration.

The control circuit may be configured to sequentially activate each of the number, M, of electric field activation sources for a first time duration while deactivating the remaining number, M, of electric field activation sources a number of times for a plurality of different time durations ranging between second and third time durations to thereby produce at the ion outlet of the drift tube ions at a number of overtone frequencies that are functionally related to the first time duration. The ion mobility spectrometer instrument may further comprise an ion detector configured to detect ions exiting the ion outlet of the first drift tube and produce corresponding ion detection signals. The control circuit may be configured to convert the ion detection signals to the frequency domain for identification of ion intensity signals at the number of overtone frequencies and at a fundamental frequency defined by the first time duration.

An ion mobility spectrometer may comprise a first drift tube partitioned into a plurality of cascaded drift tube segments each defining an ion inlet at one end and an ion outlet at an opposite end. The plurality of cascaded drift tube segments of the first drift tube may further define an ion elimination region between an ion outlet of each drift tube segment and an ion inlet of an adjacent drift tube segment. An ion source may be coupled to the ion inlet of a first one of the plurality of cascaded drift tube segments of the first drift tube. A first number, P, of electric field activation sources may each be operatively connected to one or more of the plurality of drift tube segments such that, when activated, each establishes a repulsive electric field in a different one of the first P ion elimination regions and in every following Pth ion elimination region, and also establishes an electric drift field in all remaining ion elimination regions and in all of the plurality of cascaded drift tube segments of the first drift tube. A structural change inducing device may be configured to induce structural changes in ions exiting the ion outlet of a last one of the plurality of drift tube segments of the first drift tube. A second drift tube may be partitioned into a plurality of cascaded drift tube segments each defining an ion inlet at one end and an ion outlet at an opposite end. The plurality of cascaded drift tube segments of the second drift tube may further define an ion elimination region between an ion outlet of each drift tube segment and an ion inlet of an adjacent drift tube segment. A second number, Q, of electric field activation sources may each be operatively connected to one or more of the plurality of drift tube segments of the second drift tube such that, when activated, each establishes a repulsive electric field in a different one of the first Q ion elimination regions and in every following Qth ion elimination region, and also establishes an electric drift field in all remaining ion elimination regions and in all of the plurality of cascaded drift tube segments of the second drift tube. A control circuit may be configured to sequentially activate each of the number, P, of electric field activation sources for a first time duration while deactivating the remaining number, P, of electric field activation sources a first number of times to thereby cause only ions generated by the ion source that have a first predefined ion mobility or range of ion mobilities to traverse the first drift tube. The control circuit may further be configured to sequentially activate each of the number, Q, of electric field activation sources for a second time duration while deactivating the remaining number, Q, of electric field activation sources a second number of times to thereby cause only ions exiting the structural change inducing device that have a second predefined ion mobility or range of ion mobilities to traverse the second drift tube.

The ion source may be configured to continuously generate ions. The continuously generated ions may enter the ion inlet of the first drift tube under the influence of the electric drift field sequentially established by each of the plurality, P, of electric field activation sources in the first drift tube segment of the first drift tube. Alternatively or additionally, the ion inlet of the first drift tube segment of the first drift tube may comprise an ion gate. In this embodiment, the control circuit may be configured to control the ion gate to selectively allow entrance of discrete packets of ions generated by the ion source into the ion inlet of the first drift tube. Alternatively or additionally still, the ion source may comprise at least one ion separation instrument configured to separate ions in time as a function of one or more molecular characteristics. The at least one ion separation instrument may include at least one of a liquid chromatograph, a gas chromatograph, an ion mobility spectrometer, a mass spectrometer, and a capillary electrophoresis instrument.

A method is provided for separating ions as a function of ion mobility in a drift tube partitioned into a plurality of cascaded drift tube segments each defining an ion inlet at one end and an ion outlet at an opposite end. The plurality of cascaded drift tube segments may define an ion elimination region between an ion outlet of each drift tube segment and an ion inlet of an adjacent drift tube segment. The method may comprise deactivating a second electric repulsive field and a second electric drift field and establishing a first electric repulsive field in odd-numbered ones of the ion elimination regions while also establishing a first electric drift field in even-numbered ones of the ion elimination regions and in each of the drift tube segments for a time duration, deactivating the first electric repulsive field and the first electric drift field and establishing the second electric repulsive field in even-numbered ones of the ion elimination regions while also establishing a second electric drift field in odd-numbered ones of the ion elimination regions and in each of the drift tube segments for the time duration, and sequentially repeating the deactivating steps a number of times to thereby cause only ions entering the ion inlet of a first one of the plurality of cascaded drift tube regions that have a predefined ion mobility or range of ion mobilities to traverse the drift tube.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of one illustrative embodiment of an ion mobility spectrometer instrument.

FIG. 2 is a diagram of one illustrative embodiment of the drift tube and associated arrangement of the electric field activation sources of the ion mobility spectrometer of FIG. 1.

FIG. 3 is a timing diagram illustrating operation of the ion mobility spectrometer of FIGS. 1 and 2.

FIGS. 4A is a diagram of one several cascaded sections of the drift tube illustrated in FIG. 2.

FIGS. 4B-4D illustrate alternate embodiments of the arrangement of the electric field activation sources relative to the drift tube of FIG. 4A.

FIG. 5 is a flowchart of one illustrative process for operating the ion mobility spectrometer of either of FIG. 1 or 3 as an ion mobility filter operable to produce only ions having a selected mobility or range of mobilities from a continuous or discrete ion source.

FIG. 6 is a plot of ion intensity vs. drift time illustrating the drift time of ions of the sodiated monomer [M+Na]⁺ form of the simple oligosaccharide isomer raffinose through one particular embodiment of the ion mobility spectrometer of FIG. 1.

FIG. 7 is a flowchart of one illustrative process for operating the ion mobility spectrometer of either of FIG. 1 or 3 by sweeping the pulse widths of the electric field activation sources over a range of pulse width durations.

FIG. 8 is a plot of ion intensity vs. frequency illustrating the result in the frequency domain of the process of FIG. 7 using a continuous source of raffinose ions.

FIG. 9 includes a number of plots of ion intensity vs. frequency illustrating results in the frequency domain of the process of FIG. 7 applied to ion mobility instruments having different phase numbers.

FIG. 10 includes a number of plots of ion intensity vs. frequency illustrating results in the frequency domain of the process of FIG. 5 applied to ion mobility instruments having different numbers of drift tube sections.

FIG. 11 includes a number of plots of ion intensity vs. frequency illustrating results in the frequency domain of the process of FIG. 7 applied to a raffinose sample, a melezitose sample and to a sample mixture of raffinose and melezitose.

FIG. 12A includes a number of plots of ion intensity vs. total drift time illustrating results in the time domain of the process of FIG. 5 applied to a raffinose/melezitose mixture using a cyclotron geometry ion mobility spectrometer.

FIG. 12B is a diagram of one illustrative embodiment of a cyclotron geometry ion mobility spectrometer used to generate the plots of FIG. 12A.

FIG. 13 is a block diagram of one illustrative embodiment of a cascaded ion mobility spectrometer instrument that employs some of the concepts illustrated and described with respect to FIGS. 1-12B.

DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

For the purposes of promoting an understanding of the principles of the invention, reference will now be made to a number of illustrative embodiments shown in the attached drawings and specific language will be used to describe the same.

Referring to FIG. 1, a block diagram is shown of one illustrative embodiment of an ion mobility spectrometer instrument 10. In the illustrated embodiment, the ion mobility spectrometer instrument 10 includes an ion source 12 having an ion outlet that is couple to an ion inlet of a drift tube 14. An ion outlet of the drift tube 14 is coupled to an ion detector 16 having a signal output that is electrically connected to an input of a control circuit 18. The control circuit 18 includes a conventional memory unit 20, and further includes a conventional clock circuit 22 that may be controlled by the control circuit 18 in a conventional manner to produce periodic signals of desired frequency. Illustratively, a source of buffer gas 24 supplies buffer gas to the drift tube 14 in a conventional manner.

The control circuit 18 is electrically connected to a control input of an ion source voltage supply, V_(IS), having a number, K, of outputs that are electrically connected to the ion source 12, where K may be any positive integer. The ion source 12 may be any conventional ion source that is configured to controllably produce ions from one or more samples. The ion source voltage supply, V_(IS), may accordingly represent one or more voltage supplies configured to controllably produce, under the control of the control circuit 18 and/or manually controllable or programmable, one or more corresponding voltages for controlling operation of the ion source 12 in a conventional manner to produce ions. Illustratively, the ion source 12 may be configured to continuously produce ions, or may alternatively be configured to produce discrete packets of ions. Examples of such conventional ion sources include, but are not limited to, electrospray ion sources (ESI), ion sources using radiation source to desorb ions from a sample, e.g., matrix-assisted laser desorption ion sources (MALDI), ion sources that collect generated ions in an ion trap for subsequent release, and the like. Alternatively or additionally, the ion source 12 may be or include one or more conventional ion separation instruments configured to separate ions in time as a function of one or more molecular characteristics. Examples include, but are not limited to, a conventional liquid or gas chromatograph, a conventional mass spectrometer, a conventional ion mobility spectrometer, a capillary electrophoresis instrument, or the like. In any case, ions produced by the ion source 12 exit an ion outlet of the ion source 12 and enter an ion inlet of the drift tube 14 of the ion mobility spectrometer instrument 10.

The ion mobility spectrometer 10 further includes a number, M, of electric field activation sources, e.g., voltage sources, V₁-V_(M), where M may be any positive integer greater than 1. In the illustrated embodiment, the control circuit 18 includes a corresponding number of outputs, each of which is electrically connected to an input of a different one of the electric field activation sources, V₁-V_(M). In alternate embodiments, the control circuit 18 may include fewer outputs that are electrically connected to corresponding inputs of fewer of the electric field activation sources, V₁-V_(M). In such embodiments, some of which will be described in detail hereinafter, one or more of the electric field activation sources, V₁-V_(M), may be electrically connected to corresponding outputs of the control circuit 18 and one or more others of the electric field activation sources, V₁-V_(M), may be triggered by operation of an adjacent or other ones of the electric field activation sources, V₁-V_(M), and/or be programmed for specified operation. In any case, the outputs of the voltage sources V₁-V_(M) are electrically connected to the drift tube 14 in a manner that will be fully described in detail hereinafter.

In the illustrated embodiment, the ion detector 16 is conventional and is configured to produce an ion intensity signal that is proportional to the number of ions that reach, and are detected by, the ion detector 16. The ion intensity signal is supplied to the control circuit 18, which then processes the ion intensity signal to produce ion mobility spectral information. The memory unit 20 has instructions stored therein that are executable by the control circuit 18 to control the operation of the various electric field activation sources, V₁-V_(M).

In the embodiment illustrated in FIG. 1, the drift tube 14 is partitioned into a plurality of cascaded drift tube segments beginning with a first drift tube segment positioned adjacent to the ion source 12 and defining the ion inlet of the drift tube 14, and ending with an Nth drift tube segment defining the ion outlet of the drift tube 14. Ions generated by the ion source 12 enter the ion inlet of the drift tube 14, and the electric field activation sources, V₁-V_(M), are operated such that the electric fields, i.e., drift fields, established in the various drift tube segments are modulated at a frequency that allows only ions having mobilities that are resonant with the operating conditions to drift through all of the various drift tube segments. In this way, the ion mobility spectrometer 10 operates as an ion mobility filter that filters out or away all ions except those having ion mobilities that are within a specified range of ion mobilities defined by the frequency of operation of the electric field activation sources, V₁-V_(M). Additionally, ions drift through the various drift tube segments at frequencies that are overtones of the frequency of operation of the electric field activation sources, V₁-V_(M). Thus, the ion mobility spectrometer 10 may be operated, as will be described in detail herein, to filter away all ions except those having ion mobilities that are resonant with a fundamental frequency, f_(f), and/or associated overtone frequencies, of operation of the electric field activation sources, V₁-V_(M). Because of the ability to selectively transmit ions in different frequency regions, including those associated with higher overtones, the techniques described herein may be referred to as Overtone Mobility Spectrometry (OMS). In this document, the terms “harmonics” should be understood to include the fundamental frequency, f_(f), and integer multiples of the fundamental frequency, and the term “overtone” should be understood to include only the integer multiples of the fundamental frequency, f_(f).

Referring now to FIG. 2, a diagram is shown of one illustrative embodiment of a portion of the drift tube 14 along with one illustrative arrangement of the electric field activation sources of the ion mobility spectrometer 10 of FIG. 1. In the illustrated embodiment, the drift tube 14 is partitioned into a number, N, of cascaded segments, S₁-S_(N), where N may be any positive integer greater than 2, and segments S₁-S₅ are shown in FIG. 2. Each of the segments, e.g., S₁-S₅, is illustratively constructed of five concentric, electrically conductive rings, 30 ₁-30 ₅, each separated by a concentric, electrically insulating ring 32 ₁-32 ₄ (illustrated in FIG. 2 for the segment S₁ only). The first electrically conductive ring, 30 ₁, of each segment defines an ion inlet gate, G_(I), to the segment, and the last electrically conductive ring, 30 ₅, of each segment defines an ion outlet gate, G_(O), of the segment. Illustratively, the first and last rings 30 ₁ and 30 ₅ contain mesh grids, e.g., 90% transmittance Ni mesh grid, although this disclosure contemplates embodiments which do not include one or both of the rings 30 ₁ and 30 ₅. In any case, adjacent ones of the electrically conductive rings, 30 ₁-30 ₅, are separated by one of the electrically insulating rings, 32 ₁-32 ₄, and adjacent segments, S₁-S_(N), of the drift tube 14 are separated by a concentric, electrically insulating isolator ring 34, e.g., Teflon®, a synthetic fluoropolymer resin. All of the rings, 30 ₁-30 ₅, 32 ₁-32 ₄ and 34 are stacked together, sealed with O-rings and compressed using a number, e.g., eight, of threaded rods, e.g., nylon. The segments, S₁-S_(N), are then joined together to form the drift tube 14.

In the illustrated embodiment, a resistor, R, is electrically connected between each of the electrically conductive rings in each drift tube segment, and a resistor R_(E) is connected between the ion outlet gate and ion inlet gate of each adjacent drift tube segment. The drift tube 14 of the ion mobility spectrometer instrument 10 is constructed with the electrically conductive rings 30 ₁-30 ₅ electrically insulated from each other and with the drift tube segments S₁-S_(N) also electrically isolated from each other so that electric fields can be developed separately and independently in each of the segments S₁-S_(N) and/or in groups of the segments S₁-S_(N). By applying suitable voltages across the drift tube segments and/or groups of drift tube segments, as will be described in greater detail hereinafter, uniform electric fields are illustratively established in each drift tube segment in a manner that transmits ions generated by the ion source 12 through the drift tube 14 and through the ion outlet of the last segment S_(N).

The region between the ion inlet gate and the ion outlet gate of each drift tube segment defines an ion transmission region of distance, d_(t), and the region between the ion outlet gate of one drift tube segment and the ion inlet gate of the next adjacent drift tube segment defines an ion elimination region of distance, d_(e). Thus, for example, the drift tube segment S₁ has an ion transmission region of distance, d_(t)(1) defined between G_(I1) and G_(O1), and an ion elimination region of distance d_(e)(1) defined between G_(O1) and G_(I2).

In one example embodiment, the drift tube 14 is constructed of 21 identical drift tube segments as just described, with an ion focusing funnel (not shown) positioned approximately mid way between the ion inlet and the ion outlet of the drift tube 14. In this example embodiment, the ion transmission region, d_(t), and the ion elimination region, d_(e), are together 5.84 cm in length, and the total length of the 22-section drift tube 14 is 128.5 cm from the ion inlet to the ion outlet of the drift tube 14. Further details relating to this example construction of the drift tube 14, including construction of the ion focusing funnel, are provided in co-pending U.S. Patent Application Pub. No. US 2007/0114382 A2, the disclosure of which is incorporated herein by reference. It will be understood, however, that this disclosure contemplates other embodiments in which the drift tube 14 is constructed in accordance with other conventional techniques, portions or the entirety of which may be linear or non-linear. For example, the drift tube 14 may alternatively be provided in the form of a circular or cyclotron drift tube, and further details relating to some example circular or cyclotron drift tube arrangements are provided in co-pending PCT Publication No. WO 2008/028159 A2, filed Aug. 1, 2007, the disclosure of which is incorporated herein by reference. It will be understood, however, that in any such alternate configuration the drift tube will define a number of cascaded drift tube sections such that electric fields may be selectively and separately created in individual and/or groups of the drift tube sections.

In the embodiment illustrated in FIG. 2, one illustrative arrangement 40 of the electric field activation sources, V₁-V_(M), is shown that includes two electric field activation sources, V₁ and V₂, electrically connected to the drift tube 14. The control circuit 18 is illustratively configured to control operation of the electric field activation sources, V₁ and V₂, e.g., in accordance with instructions stored in the memory 20 that are executable by the control circuit 18, in an alternating fashion to generate electric fields within the drift tube segments, S₁-S_(N), which cause ions of a specified range of ion mobilities to drift through the drift tube 14.

In the illustrated embodiment, the electric field activation sources V₁ and V₂ are both conventional DC voltage sources that are controllable by the control circuit 18 to produce a desired DC voltage across the + and − terminals. The + terminals of V₁ and V₂ are both electrically connected to the ion inlet gate, G_(I1) of the first drift tube segment, S₁. The + terminal of V₁ is further electrically connected to the ion inlet gates of the even-numbered drift tube segments, e.g., to the ion inlet gate, G_(I2) of the second drift tube segment, S₂, the ion inlet gate, G_(I4), of the fourth drift tube segment, S₄, etc., and the + terminal of V₂ is further electrically connected to the ion inlet gates of the odd-numbered drift tube segments, e.g., to the ion inlet gate, G_(I3) of the third drift tube segment, S₃, the ion inlet gate, G_(I5), of the fifth drift tube segment, S₅, etc. The − terminal of V₁ is electrically connected to the ion outlet gates of the odd-numbered drift tube segments, e.g., to the ion outlet gates, G_(O1), G_(O3), G_(O5), etc. of the drift tube segments S₁, S₃, S₅, etc., respectively. The − terminal of V₂ is electrically connected to the ion outlet gates of the even-numbered drift tube segments, e.g., to the ion outlet gates, G_(O2), G_(O4), G_(O6), etc. of the drift tube segments S₂, S₄, S₆, etc., respectively. With the exception of V₁ connected across the ion inlet and ion outlet gates, G_(I1) and G_(O1) of the first drift tube segment, S₁, V₁ and V₂ are thus connected across the ion inlet and outlet gates of alternating, adjacent pairs of drift tube segments. For example, V₂ is electrically connected across S₁ and S₂, e.g., between G_(I1) and G_(O2), V₁ is electrically connected across S₂ and S₃, e.g., between G_(I2) and G_(O3), V₂ is electrically connected across S₃ and S₄, e.g., between G_(I3) and G_(O4), etc.

As illustrated in FIG. 2, the voltage sources, V₁ and V₂, when activated, produce linear voltage gradients, VG₁ and VG₂ respectively, across the drift tube segments to which they are connected. Equal-valued resistors, R, are electrically connected across adjacent pairs of the electrically conductive rings, 30 ₁-30 ₅, of each drift tube segment, S₁-S_(N), and equal-valued resistors, R_(E), are connected between the ion outlet gates and ion inlet gates of adjacent pairs of drift tube segments. The value of R_(E) is selected relative to R (or vice versa) such that the linear voltage gradients, VG₁ and VG₂, establish corresponding, constant-valued electrical fields across the various drift tube segments pairs.

The control circuit 18 is configured to control operation of the voltage sources, V₁ and V₂, by periodically switching one voltage source, V₁, V₂, on while the other voltage source, V₁, V₂, is off. This has the effect of alternatively establishing a constant electric field across sequential, cascaded pairs of the drift tube segment, S₁-S_(N). This generally allows only ions having ion mobilities that match the switching frequency to traverse each cascaded pair of drift tube segments. The periodic switching between V₁ and V₂ also establishes a repulsive electric field, i.e., an electric field that is oriented to repel ions traveling in a direction toward the ion detector 16, in the ion elimination regions, d_(e), that follow each cascaded pair of drift tube segments. To illustrate this repulsive electric field, consider the case when V₁ is off and V₂ is on so that only the voltage gradients VG₂ of FIG. 2 exist. Ions entering the first drift tube segment S₁ will drift under the constant electric field established by VG₂ while V₂ is on. However, ions that reach G_(O2) while V₂ is still on will be filtered out by the repulsive electric field, e.g., reverse electric field, established between the high +V₂ potential at the ion inlet gate G_(I3) and the low −V₂ potential at the ion outlet gate G_(O2). Generally, V₂ establishes, when activated, repulsive electric fields in the ion elimination regions d_(e) between the ion outlet gates of even-numbered drift tube segments and the ion inlet gates of the next sequential, odd-numbered drift tube segments, and V₁ likewise establishes, when activated, identical repulsive electric fields in the ion elimination regions d_(e) between the ion outlet gates of odd-numbered drift tube segments and the ion inlet gates of the next sequential, even-numbered drift tube segments. This periodic traversal of two drift tube segments and ion elimination in the activated ion elimination regions, d_(e), causes only ions having ion mobilities that drift in the established electric fields at the rate defined by the V₁, V₂ switching rate and overtones thereof to drift through the length of the drift tube 14 to the ion detector 16. Generally, if the switching rate between V₁ and V₂ is constant, this switching rate defines a fundamental frequency, f_(f), at which ions of a corresponding range of mobilities can travel progressively through the drift tube segments S₁-S_(N). Alternatively or additionally, if the switching rate is swept over a range of switching rates, ions having the corresponding range of ion mobilities will also travel progressively through the drift tube segments S₁-S_(N) at overtone frequencies of the fundamental frequency, f_(f).

Referring now to FIG. 3, a number of plots A-F are shown demonstrating the progression of ions through the first five segments, S₁-S₅ of the drift tube 14 of FIGS. 1 and 2 when the voltage sources V₁ and V₂ are alternatively switched on and off. In the example illustrated in FIG. 5, the ion source 12 is configured to continually produce ions 50. Plot A illustrates the condition when V₁ and V₂ are both initially off. Plot B illustrates the condition when V₁ is subsequently turned on while V₂ remains off, which establishes a constant-valued electric field, E₁, in the ion transmission regions d_(t) of each of the drift tube segments, S₁-S₅, and also in the even-numbered ion elimination regions d_(e)(2) and d_(e)(4), and which establishes a repulsive electric field in the odd-numbered ion elimination regions d_(e)(1), d_(e)(3) and d_(e)(5). A portion 51 of the continually generated ions 50 drift through d_(t)(1) under the influence of the electric field E₁ toward d_(t)(2). However, ions that arrive at the ion elimination region d_(e)(1) before V₁ is switched off are filtered out of the ions 51 by the repulsive field established in the ion elimination region d_(e)(1) by V₁. It will be understood that the voltage applied by V₁ across the first drift tube segment, S₁, will be different than that applied across remaining pairs of the drift tube segments as illustrated in FIG. 2. Generally, the voltage applied by V₁ across the first drift tube segment, S₁, will be selected so as to establish an electric field, E₁, in the ion transmission region d_(t)(1) that is identical to the electric field E₁ established across various pairs of the remaining drift tube segments, S₂-S_(N).

Plot C illustrates the condition when V₁ is switched off and V₂ is switched on, which establishes a constant-valued electric field, E₂ (E₂=E₁), in the ion transmission regions d_(t) of each of the drift tube segments, S₁-S₅, and also in the odd-numbered ion elimination regions d_(e)(1), d_(e)(3) and d_(e)(5), and which establishes a repulsive electric field in the even-numbered ion elimination regions d_(e)(2) and d_(e)(4). The portion of ions 51 in the d_(t)(1) region continues to advance under the influence of the electric field E₂ through d_(t)(2) toward d_(t)(3), and another portion 52 of the continually generated ions 50 drifts through d_(t)(1) under the influence of the electric field E₂ toward d_(t)(2). Ions that arrive at the ion elimination region d_(e)(2) before V₂ is switched off are filtered out of the ions 51 by the repulsive field established in the ion elimination region d_(e)(2) by V₂.

Plot D illustrates the condition when V₂ is switched off and V₁ is switched back on, which establishes the constant-valued electric field E₁ as described with respect to plot B. The portion of ions 51 in the d_(t)(2) region continues to advance under the influence of the electric field E₁ through d_(t)(3) toward d_(t)(4), and another portion 53 of the continually generated ions 50 drifts through d_(t)(1) under the influence of the electric field E₁ toward d_(t)(2). However, the ions 52 that were previously in the d_(t)(1) region are filtered away by the repulsive electric field established in the ion elimination region d_(e)(1) and therefore do not advance to d_(t)(2), and ions that arrive at the ion elimination regions d_(e)(1) and d_(e)(3) before V₁ is switched off are filtered out of the ions 53 and 51 respectively by the repulsive field established in the ion elimination regions d_(e)(1) and d_(e)(3) respectively.

Plot E illustrates the condition when V₁ is again switched off and V₂ is switched back on, which establishes the constant-valued electric field, E₂ described with respect to plot C. The portion of ions 51 in the d_(t)(3) region continues to advance under the influence of the electric field E₂ through d_(t)(4) toward d_(t)(5), the portion of ions 53 in the d_(t)(1) region continues to advance under the influence of the electric field E₂ through d_(t)(2) toward d_(t)(3), and yet another portion 54 of the continually generated ions 50 drifts through d_(t)(1) under the influence of the electric field E₂ toward d_(t)(2). Ions that arrive at the ion elimination regions d_(e)(2) and d_(e)(4) before V₂ is switched off are filtered out of the ions 53 and 51 respectively by the repulsive field established in the ion elimination regions d_(e)(2) and d_(e)(4) respectively.

Plot F illustrates the condition when V₂ is again switched off and V₁ is switched back on, which establishes the constant-valued electric field E₁ as described with respect to plot B. The portion of ions 51 in the d_(t)(4) region continues to advance under the influence of the electric field E₁ through d_(t)(5) toward the next drift tube segment (S₆), the portion of ions 53 in the d_(t)(2) regions continues to advance under the influence of the electric field E₁ through d_(t)(3) toward d_(t)(4), and yet another portion 55 of the continually generated ions 50 drifts through d_(t)(1) under the influence of the electric field E₁ toward d_(t)(2). The ions 54 that were previously in the d_(t)(1) region are filtered away by the repulsive electric field established in the ion elimination region d_(e)(1) and therefore do not advance to d_(t)(2), and ions that arrive at the ion elimination regions d_(e)(3) and d_(e)(5) before V₁ is switched off are filtered out of the ions 53 and 51 respectively by the repulsive fields established in the ion elimination regions d_(e)(3) and d_(e)(5) respectively.

While the embodiment of the drift tube 14 of FIG. 2 was illustrated and described as including an arrangement 40 of electric field activation sources in the form of two DC voltage sources, V₁ and V₂, it will be understood that this disclosure is not so limited and that embodiments are contemplated in which the arrangement of electric field activation sources includes more than two voltage sources. Referring now to FIGS. 4A-4D, for example, a number of voltage gradient plots are shown, in relation to the first eight cascaded segments, S₁-S₈, of the drift tube 14, that illustrate alternative embodiments in which the arrangement of electric field activation sources include additional voltage sources. As a reference, the voltage gradient plot 40 of FIG. 4B, illustrates the embodiment just described in which the arrangement of electric field activation sources includes two voltage sources V₁ and V₂ connected and configured to produce the two illustrated voltage gradients VG₁ and VG₂.

The voltage gradient plot 60 of FIG. 4C, in contrast, illustrates an embodiment in which the arrangement of electric field activation sources includes three voltage sources, V₁, V₂ and V₃, each illustratively identical to the voltage sources V₁ and V₂ illustrated and described with respect to FIG. 2. In the embodiment of FIG. 4C, +V₁, +V₂ and +V₃ are all electrically connected to the ion inlet gate G_(I1), of the first drift tube segment, S₁. The +V₁ is further electrically connected to the ion inlet gates G_(I2), G_(I5) and G_(I8), of the drift tube segments S₂, S₅ and S₈ respectively, the +V₂ is further electrically connected to the ion inlet gates G_(I3), and G_(I6), of the drift tube segments S₃ and S₆ respectively, and the +V₃ is further electrically connected to the ion inlet gates G_(I4), and G_(I4), of the drift tube segments S₄ and S₇ respectively. The −V₁ is electrically connected to the ion outlet gates G_(O1), G_(O4) and G_(O7), the −V₂ is electrically connected to the ion outlet gates G_(O2), G_(O5) and G_(O8), and the −V₃ is electrically connected to the ion outlet gates G_(O3) and G_(O6). In the three voltage source arrangement, the voltage sources V₁-V₃ are thus electrically connected, in alternating fashion, across three consecutive drift tube segments with all three voltage sources electrically connected to the ion inlet grid G_(I1) of the first drift tube segment, S₁, and then with V₁ electrically connected across S₁, S₂-S₄, and S₅-S₇, with V₂ electrically connected across S₁-S₂, S₃-S₅ and S₆-S₈ and with V₃ electrically connected across S₁-S₃ and S₄-S₇.

In operation, the control circuit 18 controls the voltage sources V₁-V₃ by sequentially switching one voltage source on for a specified duration while maintaining the other two voltage sources in their off state for that duration. As illustrated in FIG. 4C, for example, the control circuit 18 turns on V₁ for the specified duration while maintaining V₂ and V₃ in their off states, followed by turning off V₁ and turning on V₂ while maintaining V₃ in its off state for the specified duration, followed by turning off V₂ and turning on V₃ while maintaining V₁ in its off state for the specified duration. The control circuit 18 repeats the above process many times to cause ions having mobilities related to the voltage source switching frequency to drift through the various drift tube segments. It will be understood that the voltage applied by V₁ across the first drift tube segment, S₁, will be different than that applied by V₁ across remaining triplets of the drift tube segments, and that the voltage applied by V₂ across the first two drift tube segments, S₁-S₂, will also be different than that applied by V₂ across remaining triplets of the drift tube segments. Generally, the voltage applied by V₁ across the first drift tube segment, S₁, will be selected so as to establish an electric field, E₁, in the ion transmission region d_(t)(1) that is identical to the electric field E₁ established by V₁ across various triplets of the remaining drift tube segments, S₂-S_(N), and the voltage applied by V₂ across the first two drift tube segment, S₁-S₂, will be selected so as to establish an electric field, E₂, in the ion transmission region d_(t)(1), ion elimination region d_(e)(1) and ion transmission region d_(t)(2) that is identical to the electric field E₂ established by V₂ across various triplets of the remaining drift tube segments, S₃-S_(N).

The voltage gradient plot 70 of FIG. 4D illustrates an embodiment in which the arrangement of electric field activation sources includes four voltage sources, V₁, V₂, V₃ and V₄, each illustratively identical to the voltage sources V₁ and V₂ illustrated and described with respect to FIG. 2. In the embodiment of FIG. 4D, +V₁, +V₂, +V₃ and +V₄ are all electrically connected to the ion inlet gate G_(I1) of the first drift tube segment, S₁. The +V₁ is further electrically connected to the ion inlet gates G_(I2) and G_(I6), of the drift tube segments S₂ and S₆ respectively, the +V₂ is further electrically connected to the ion inlet gates G_(I3), and G_(I7), of the drift tube segments S₃ and S₇ respectively, the +V₃ is further electrically connected to the ion inlet gates G_(I4), and G_(I8), of the drift tube segments S₄ and S₈ respectively, and the +V₄ is further electrically connected to the ion inlet gate G_(I5) of the drift tube segment S₅. The −V₁ is electrically connected to the ion outlet gates G_(O1) and G_(O5), the −V₂ is electrically connected to the ion outlet gates G_(O2) and G_(O6), the −V₃ is electrically connected to the ion outlet gates G_(O3) and G_(O7), and the −V₄ is electrically connected to the ion outlet gates G_(O4) and G_(O8). In the four voltage source arrangement, the voltage sources V₁-V₄ are thus electrically connected, in alternating fashion, across four consecutive drift tube segments with all four voltage sources electrically connected to the ion inlet grid G_(I1) of the first drift tube segment, S₁, and then with V₁ electrically connected across S₁, S₂-S₅, and S₆-S₉, with V₂ electrically connected across S₁-S₂ and S₃-S₆, with V₃ electrically connected across S₁-S₃ and S₄-S₇, and with V₄ electrically connected across S₁-S₄ and S₅-S₈.

In operation, the control circuit 18 controls the voltage sources V₁-V₄ by sequentially switching one voltage source on for a specified duration while maintaining the other three voltage sources in their off state for that duration. As illustrated in FIG. 4D, for example, the control circuit 18 turns on V₁ for the specified duration while maintaining V₂, V₃ and V₄ in their off states, followed by turning off V₁ and turning on V₂ while maintaining V₃ and V₄ in their off states for the specified duration, followed by turning off V₂ and turning on V₃ while maintaining V₁ and V₄ in their off states for the specified duration, followed by turning off V₃ and turning on V₄ while maintaining V₁ and V₂ in their off states for the specified duration. The control circuit 18 repeats the above process many times to cause ions having mobilities related to the voltage source switching frequency to drift through the various drift tube segments. It will be understood that the voltage applied by V₁ across the first drift tube segment, S₁, will be different than that applied by V₁ across remaining quadruplets of the drift tube segments, the voltage applied by V₂ across the first two drift tube segments, S₁-S₂, will be different than that applied by V₂ across remaining quadruplets of the drift tube segments, and the voltage applied by V₃ across the first three drift tube segments, S₁-S₃, will be different than that applied by V₃ across remaining quadruplets of the drift tube segments. Generally, the voltage applied by V₁ across the first drift tube segment, S₁, will be selected so as to establish an electric field, E₁, in the ion transmission region d_(t)(1) that is identical to the electric field E₁ established by V₁ across various quadruplets of the remaining drift tube segments, S₂-S_(N), the voltage applied by V₂ across the first two drift tube segment, S₁-S₂, will be selected so as to establish an electric field, E₂, in the ion transmission region d_(t)(1), ion elimination region d_(e)(1) and ion transmission region d_(t)(2) that is identical to the electric field E₂ established by V₂ across various quadruplets of the remaining drift tube segments, S₃-S_(N), and the voltage applied by V₃ across the first three drift tube segment, S₁-S₃, will be selected so as to establish an electric field, E₃, in the ion transmission regions d_(t)(1), d_(t)(2) and d_(t)(3) and in the ion elimination regions d_(e)(1) and d_(e)(2) that is identical to the electric field E₃ established by V₃ across various quadruplets of the remaining drift tube segments, S₄-S_(N).

The number of electric field activation sources, e.g., voltage sources, used in any particular embodiment, and the manner in which they are electrically connected to the various drift tube segments to operate as described above, is referred to as the phase (φ) of the ion mobility spectrometer 10. In the example of FIGS. 2, 3 and 4B in which two voltage sources V₁ and V₂ are used as described above, φ=2 as indicated in the plot 40 of FIG. 4B. In the example of FIG. 4C in which three voltage sources V₁, V₂ and V₃ are used as described above, φ=3 as indicated in the plot 60 of FIG. 4C. In the example of FIG. 4D in which four voltage sources V₁, V₂, V₃ and V₄ are used as described above, φ=4 as indicated in the plot 70 of FIG. 4D. It will be noted from FIG. 3 that in a φ=2 system, the fill rate of ions in the various drift tube segments S₁-S_(N), i.e., the duty cycle of the ion mobility spectrometer 10, is 50%. It can be shown that in a φ=3 system, the duty cycle of the ion mobility spectrometer is 66.67% and in a φ=4 system, the duty cycle of the ion mobility spectrometer is 75%. A general expression for the duty cycle, d, of the ion mobility spectrometer 10 as a function of the phase, φ, is thus d=1−(1/φ).

Referring again to FIG. 3, the electric fields in the drift tube 14 in a two-phase (φ=2) ion mobility spectrometer 10 are established by two source V₁ and V₂. The electric fields established in the drift tube 14 by activation of V₁ include an electric drift field E₁, i.e., an electric field through which ions generated by the ion source drift toward the ion detector 16, in each of the drift tube segments, i.e., in each of the ion transmission regions d_(t), and also in the even-numbered ion elimination regions, d_(e)(2), d_(e)(4), etc., and also includes a repulsive electric field, i.e., an electric field that repels and filters away ions traveling in the direction of the electric drift field, in odd-numbered ion elimination regions, d_(e)(1), d_(e)(3), d_(e)(5), etc. Similarly, the electric fields established in the drift tube 14 by activation of V₂ includes an electric drift field E₂ in each of the drift tube segments and also in the odd-numbered ion elimination regions, d_(e)(1), d_(e)(3), d_(e)(5), etc., and also includes a repulsive electric field in even-numbered ion elimination regions, d_(e)(2), d_(e)(4), etc.

It can be shown that in three-phase systems (φ=3) that include three electric field activation sources, V₁-V₃, such as that illustrated in FIG. 4C, activation of V₁ establishes a repulsive electric field in the first ion elimination region, d_(e)(1), and in every following 3^(rd) ion elimination region, d_(e)(4), d_(e)(7), d_(e)(10), etc., and also establishes an electric drift field, E₁, in all remaining ion elimination regions and in all of the drift tube segments d_(t). Activation of V₂ likewise establishes a repulsive electric field in the second ion elimination region, d_(e)(2), and in every following 3^(rd) ion elimination region, d_(e)(5), d_(e)(8), d_(e)(11), etc., and also establishes an electric drift field, E₂, in all remaining ion elimination regions and in all of the drift tube segments d_(t). Activation of V₃ similarly establishes a repulsive electric field in the third ion elimination region, d_(e)(3), and in every following 3^(rd) ion elimination region, d_(e)(6), d_(e)(9), d_(e)(12), etc., and also establishes an electric drift field, E₃, in all remaining ion elimination regions and in all of the drift tube segments d_(t).

It can also be shown that in four-phase systems (φ=4) that include four electric field activation sources, V₁-V₄, such as that illustrated in FIG. 4D, activation of V₁ establishes a repulsive electric field in the first ion elimination region, d_(e)(1), and in every following 4^(th) ion elimination region, d_(e)(5), d_(e)(9), d_(e)(13), etc., and also establishes an electric drift field, E₁, in all remaining ion elimination regions and in all of the drift tube segments d_(t). Activation of V₂ likewise establishes a repulsive electric field in the second ion elimination region, d_(e)(2), and in every following 4^(th) ion elimination region, d_(e)(6), d_(e)(10), d_(e)(14), etc., and also establishes an electric drift field, E₂, in all remaining ion elimination regions and in all of the drift tube segments d_(t). Activation of V₃ similarly establishes a repulsive electric field in the third ion elimination region, d_(e)(3), and in every following 4^(th) ion elimination region, d_(e)(7), d_(e)(11), d_(e)(15), etc., and also establishes an electric drift field, E₃, in all remaining ion elimination regions and in all of the drift tube segments d_(t). Finally, activation of V₄ establishes a repulsive electric field in the fourth ion elimination region, d_(e)(4), and in every following 4^(th) ion elimination region, d_(e)(8), d_(e)(12), d_(e)(16), etc., and also establishes an electric drift field, E₄, in all remaining ion elimination regions and in all of the drift tube segments d_(t).

From the foregoing examples, a generalized characterization can be made for an M-phase system, i.e., one that includes a number, M, of electric field activation sources, V₁-V_(M). In such an M-phase system, the number, M, of electric field activation sources are each be operatively connected to one or more of the plurality of drift tube segments such that, when activated, each establishes a repulsive electric field in a different one of the first M ion elimination regions and in every following Mth ion elimination region, and also establishes an electric drift field in all remaining ion elimination regions and in all of the plurality of cascaded drift tube segments. In operation, the control circuit 18 sequentially activates each of the number, M, of electric field activation sources for a time duration while deactivating the remaining number, M, of electric field activation sources a number of times to thereby cause only ions generated by the ion source that have a predefined ion mobility or range of ion mobilities to traverse the drift tube 14.

Transmission of ions through the various drift tube segments S₁-S_(N) as just described is only possible if the mobilities of the ions are in resonance with the switching rates of the electric fields applied by the electric field activation sources regardless of the phase of the spectrometer 10. In other words, to transmit ions sequentially through the various drift tube segments S₁-S_(N) as just described, the ions must have mobilities that allow traversal exactly one drift tube segment in one field application duration. Ions with mobilities that are off resonance either traversing a drift tube segment too quickly or too slowly are eventually eliminated in one of the ion elimination regions d_(e). The frequency at which the various electric field activation sources are switched on/off, i.e., the frequency at which the ions have resonant mobilities, is termed the fundamental frequency, f_(f).

In the above description of the operation of the ion mobility spectrometer instrument 10, the control circuit 18 is described as being configured to control operation of the electric field activation sources V₁-V_(M). Illustratively, the memory unit 20 has instructions stored therein that are executable by the control circuit 18 to control operation of the electric field activation sources V₁-V_(M) in this manner.

In one alternative embodiment, each of the electric field activation sources V₁-V_(M) may be programmable to produce, when triggered by an adjacent, e.g., lower-numbered, one of the electric field activation sources V₁-V_(M), an electric field activation pulse of desired duration. In this embodiment, each higher-numbered one of the electric field activation sources V₁-V_(M) may be programmable to be triggered for activation by deactivation of an adjacent lower-numbered one of the electric field activation sources V₁-V_(M). Thus, deactivation of V₁ will trigger activation of V₂, deactivation of V₂ will trigger activation of V₃ (or V₁ again), and so forth. In this embodiment, the control circuit 18 is configured to control operation of the electric field activation sources V₁-V_(M) only by activating the first one of the electric field activation sources V₁-V_(M).

In another alternative embodiment, each of the electric field activation sources V₁-V_(M) may be programmable to produce, when triggered by the control circuit 18, an electric field activation pulse having desired duration. In this embodiment, the control circuit 18 controls activation times of each of the electric field activation sources V₁-V_(M), and once activated each of the electric field activation sources V₁-V_(M) is operable to produce a pulse having time duration equal to a programmed pulse duration. In this embodiment, the control circuit 18 is configured to control operation of the electric field activation sources V₁-V_(M) only by activating at specified times each of the electric field activation sources V₁-V_(M).

It will be understood that while the electric field activation sources, V₁-V_(M) were described as producing DC voltages of programmable duration this disclosure contemplates embodiments in which the electric field activation sources are configured to produce alternatively shaped electric field activation pulses. For example, such alternatively shaped electric field activation pulses may be linear or piece-wise linear pulse shapes, such as triangular or other linear or piece-wise linear shapes, or may be non-linear shapes such as sine-wave, Gaussian or other non-linear shapes. The corresponding electric fields applied in time-dependent fashion to the various segments S₁-S_(N) of the drift tube 14 as described above may thus be linearly, piece-wise linearly or non-linearly varying. Alternatively still, different ones and/or blocks of the electric field activation sources V₁-V_(M) may be activated for different durations. Those skilled in the art will recognize that, in general, any one or more of the segments S₁-S_(N) may be operated for a duration that is different than the duration of operation of any one or more of the remaining ones of the segments S₁-S_(N), and that operation of the ion mobility instrument 10 in this manner will result in a multi-dimensional ion mobility spectrometer instrument, i.e., a drift tube having one or more segments in any location relative to the ion inlet and ion outlet that is/are tuned to pass therethrough only ions having a mobility or range of mobilities that is/are different than that/those of one or more of the remaining segments.

Referring now to FIG. 5, a flowchart is shown of one illustrative embodiment of a process 80 for operating the ion mobility spectrometer instrument 10 to act as an ion mobility filter by allowing travel through the drift tube 14 of only ions having a predefined mobility or range of mobilities as described hereinabove with respect to FIGS. 1-4D. The process 80 illustrated in FIG. 5 may be provided, in whole or in part, in the form of instructions that are stored in the memory unit 20 of the control circuit 18 and that are executable by the control circuit 18 to control the ion mobility spectrometer instrument 10 in accordance with the process 80. The process 80 begins at step 82 where the drift time, DT, through the drift tube 14 is determined for the sample component of interest. This may be done, for example, by first operating the ion drift tube 14 in a conventional manner to determine the drift time, DT, of the sample component of interest, although other conventional techniques may be used at step 82 to determine the drift time, DT, of the sample component of interest. For example, conventional ion mobility spectrometer configurations could be used to determine a drift time, or a drift time could be retrieved from literature, although in either case, instrument operating parameters used to determine such drift times, e.g., buffer gas pressure, operating temperature, drift tube length, etc., would have to be taken into account to determine corresponding operating parameters for those of the ion mobility spectrometer 10. Referring to FIG. 6, a plot 94 of ion intensity vs. drift time is shown illustrating the drift time of ions of the sodiated monomer [M+Na]⁺ form of the simple oligosaccharide isomer raffinose through one particular embodiment of the ion mobility spectrometer 10. In this embodiment, the drift time, DT, of raffinose through the drift tube 14 is approximately 15.8 ms.

Following step 82, the pulse width, PW, of the number of electric field activation sources V₁-V_(M) used in the ion mobility spectrometer instrument 10 is computed at step 84. The pulse width, PW, is the duration of the electric field that will be applied by each of the electric field activation sources V₁-V_(M) to pass the sample component of interest, e.g., raffinose, through each segment. In order to pass the sample component of interest through a drift tube 14 having N segments, the pulse width, PW, must therefore satisfy the relationship PW=DT/N.

Following step 84, the shape of the pulse width is selected at step 86, and thereafter at step 88 the peak voltage of the electric field activation sources V₁-V_(M) is selected. The process 80 advances from step 88 to step 90, and simultaneously with step 90 the ion source voltage supply, V_(IS), is controlled at step 92 in a manner that causes the ion source 12 to produce ions. The ions produced at step 92 may be produced continuously or may instead be produced discretely as described hereinabove. In any case, the control circuit 18 is operable at step 90 to control the electric field activation sources V₁-V_(M), as described hereinabove, to sequentially apply electric fields having the selected shape, duration and peak field strength to the various drift tube segments, S₁-S_(N) as described hereinabove by example with reference to FIGS. 2-4D. Steps 90 and 92 may be repeated continuously or a finite number of times to thereby operate the ion mobility spectrometer instrument 10 as a continuous or discrete ion mobility filter. For the raffinose sample illustrated in FIG. 6, the pulse width, PW, of the electric field activation sources V₁-V_(M) in the ion mobility spectrometer instrument 10 of FIG. 1, in which the drift tube 14 is constructed of 20 drift tube segments, S₁-S₂₀, and one ion focusing filter positioned approximately mid way between the ion inlet and ion outlet of the drift tube 14, is approximately 500 microseconds, which corresponds to an electric field activation source switching frequency of approximately 2.0 kHz. It will be understood that steps 82-88 are not required to be executed in the illustrated order, and that one or more of these steps may alternatively be interchanged with one or more other of these steps.

In addition to operation of the ion mobility spectrometer instrument 10 as an ion mobility filter as illustrated and described herein, the ion mobility spectrometer instrument 10 may also be operated in a manner that transmits ions at overtone frequencies of the fundamental frequency, f_(f), as described briefly above. Referring to FIG. 7, for example, a flowchart of an illustrative process 100 for operating the ion mobility spectrometer 10 by sweeping the pulse widths, PW, of the electric field activation sources, V₁-V_(M), over a range of pulse width durations. In addition to producing a fundamental ion intensity peak that corresponds to ion intensity peak resulting from the process 80 of FIG. 5, the process 100 further produces overtone ion intensity peaks that may be analyzed in the frequency domain to reveal additional characteristics of the sample component of interest. The process 100 illustrated in FIG. 7 may be provided, in whole or in part, in the form of instructions that are stored in the memory unit 20 of the control circuit 18 and that are executable by the control circuit 18 to control the ion mobility spectrometer instrument 10 in accordance with the process 100.

The process 100 begins at step 102 where initial and final pulse width durations, PW_(I) and PW_(F) respectively, and a ramp rate, R (or step size), are selected. Illustratively, the initial pulse width duration, PW_(I), may be selected to be slightly longer than necessary to produce the fundamental ion intensity peak so that the resulting ion intensity vs. frequency spectrum begins approximately at the fundamental peak. Illustratively, the final pulse width duration, PW_(F), may be selected to be a frequency beyond which no useful information is expected to occur, or beyond which no ion intensity information is sought. In any case, the ramp rate, R, and/or frequency step size between the initial and final pulse width durations, PW_(I) and PW_(F), will typically be selected to provide sufficient time at each pulse width duration to extract useful information from the ion mobility spectrometer instrument 10.

Following step 102, the shape of the pulse width is selected at step 104, and thereafter at step 106 the peak voltage of the electric field activation sources V₁-V_(M) is selected. The process 100 advances from step 106 to step 108, and simultaneously with step 108 the ion source voltage supply, V_(IS), is controlled at step 110 in a manner that causes the ion source 12 to produce ions. The ions produced at step 110 may be produced continuously or may instead be produced discretely as described hereinabove, although if produced discretely a timing mechanism will typically be required to trigger new supplies of ions coincident with the changing of the pulse width durations. In any case, the control circuit 18 is operable at step 108 to control the electric field activation sources V₁-V_(M), as described hereinabove, to apply electric fields having the selected shape and peak field strength to the drift tube segments, S₁-S_(N) while sweeping the pulse width duration, PW, between PW_(I) and PW_(F) at the selected ramp rate and/or step size. It will be understood that steps 102-106 are not required to be executed in the illustrated order, and that one or more of these steps may alternatively be interchanged with one or more other of these steps.

While not specifically illustrated in FIG. 7 as a step in the process 100, ion detection signals produced by the ion detector 16 may be processed by the control circuit 18 and converted to the frequency domain in a conventional manner for further analysis and/or observation. For the raffinose sample illustrated in FIG. 6, for example, sweeping the pulse width, PW, of the electric field activation sources V₁-V₄ in the ion mobility spectrometer instrument 10 of FIG. 1 between approximately 10 milliseconds down to approximately 22 micro-seconds yields the ion intensity vs. frequency spectrum 112 illustrated in FIG. 8. Illustratively, the 3^(rd) overtone produces the most highly resolved ion intensity peak.

Generally, the overtones produced by frequency sweeps of the type illustrated in FIG. 8 will be defined, at least in part, by the phase (φ) of the ion mobility spectrometer 100. Referring to FIG. 9, for example, a number of plots 120, 122, 124, 126 and 128 are shown of frequency spectrums of raffinose in which the phase, φ, of the ion mobility spectrometer 10 correspondingly increases. In all cases, the electric field activation sources, V₁-V_(M) were configured to operate as described hereinabove with respect to FIGS. 2-4D. In the plot 120, φ=2, and the associated frequency spectrum includes an ion peak at the fundamental frequency, f_(f), and additional peaks at the third and fifth overtones. In the plot 122, φ=3, and the associated frequency spectrum includes an ion peak at the fundamental frequency, f_(f), and additional peaks at the fourth and seventh overtones. In the plots 124, 126 and 128, φ=4, 5 and 6 respectively. The frequency spectrum 124 includes an ion peak at the fundamental frequency, f_(f), and additional peaks at the fifth, ninth and thirteenth overtones, the frequency spectrum 126 includes an ion peak at the fundamental frequency, f_(f), and additional peaks at the sixth, eleventh and sixteenth overtones, and the frequency spectrum 128 includes an ion peak at the fundamental frequency, f_(f), and additional peaks at the seventh, thirteenth and nineteenth overtones. It should be noted that as the phase of the ion mobility spectrometer 10 is increased, secondary overtones increasing appear between the fundamental peak, f_(f), and the first expected overtone. These secondary overtones correspond to intermediate harmonic frequencies, i.e., those between the overtone frequencies, and may carry additional ion information.

Generally, the overtones that should be expected to be observed in embodiments of the ion mobility spectrometer 10 operated with uniform, constant electric fields in the various drift tube segments, S₁-S_(N) as described hereinabove, are given by the equation H=φ(h−1)+1, h=1, 2, 3, . . . , where H is a harmonic number, φ is the phase of the ion mobility spectrometer 10, and h is an integer. Thus, for φ=2, H=1, 3, 5, 7, . . . , for φ=3, H=1, 4, 7, 10, . . . , for φ=4, H=1, 5, 9, 13, . . . , for φ=5, H=1, 6, 11, 16, . . . , and for φ=6, H=1, 7, 13, 19, . . . , etc.

The resolving power, R, of the ion mobility spectrometer instrument 10 is defined by the equation R_(OMS)=f/Δf, where f is the frequency at which maximum ion intensity of transmitted and Δf is the width of the peak at half maximum. Generally, it is observed that the resolving power, R, of the ion mobility spectrometer instrument 10 increases with increasing overtone number, H. The resolving power, R, of the ion mobility spectrometer instrument 10 is also influenced by the total number, N, of drift tube segments, S₁-S_(N), used. Referring to FIG. 10, for example, a number of plots 130 are shown illustrating the shapes of fundamental-frequency, ion intensity peaks for a four-phase (φ=4) ion mobility spectrometer instrument 10 in which the total number of drift tube segments, N, is varied between 11 and 43 as indicated on the left portion of FIG. 10. The plots 130 were generated from a sample of the sodiated monomer [M+Na]⁺ form of the simple oligosaccharide isomer melezitose using the ion mobility instrument 10 illustrated and described herein. As illustrated in FIG. 10, whereas the peak intensities do not change significantly, the widths, Δf, of the peaks at half maximum decrease as N increases. For example, Δf₁₁, corresponding to the width of the N=11 peak at half maximum, is approximately 1600 Hz, whereas Δf₄₃, corresponding to the width of the N=43 peak at half maximum, is approximately 345 Hz. The ratio f/Δf, and thus, R_(OMS), accordingly increases as the number, N, of drift tube segments S₁-S_(N) increases.

In a conventional ion mobility spectrometer; that is to say, an ion mobility spectrometer in which a single electric field is applied across the length of the drift tube, the resolving power is generally understood to follow the relationship R_(IMS)=SQRT[(E*e*L)/(16*k_(b)*T*ln 2)], where E is the applied electric field, e is the elementary charge value, L is the length of the drift tube, k_(b) is Boltzmann's constant, and T is the temperature of the drift tube.

In the ion mobility instruments 10 illustrated and described herein, the overall resolving power, R_(OMS), is a function of R_(IMS) and is also a function of the total number, N, of drift tube segments, S₁-S_(N), the phase number, φ, of the applied electric field and the harmonic number, H. Illustratively, R_(OMS) is given by the equation: R_(OMS)=1/[(C/R_(IMS))*{1−[φ−1−d_(e)/(d_(t)+d_(e))]/N*H}*{[φ−1−d_(e)/(d_(t)+d_(e))]/N*H}], where C is a constant and all other variables have been defined herein. It should be noted that the resolving power, R_(OMS), generally increases with increasing N and also with increasing H, and the resolving power, R_(OMS), decreases with increasing φ. It should also be noted that in the limit of high R_(IMS), the first term in the denominator of the foregoing equation approaches zero and the foregoing equation reduces to R_(OMS)=N*H/[φ−1−d_(e)/(d_(t)+d_(e))].

Referring now to FIG. 11, plots of ion intensity vs. frequency are shown to illustrate one implementation of the enhanced resolving power of the ion mobility spectrometer 10 at overtone frequencies. The plots of FIG. 11 are frequency-domain plots that were generated with a two-phase (φ=2) configuration of the ion mobility instrument 10. In the plots of FIG. 11, only the overtone peaks 3 and 5 are shown along with the peak at the fundamental frequency, 1. The plot 140 represents a frequency-domain plot of raffinose, the plot 142 represents a frequency-domain plot of the sodiated monomer [M+Na]⁺ form of the simple oligosaccharide isomer melezitose, and the plot 144 represents a frequency-domain plot of a 3:1 raffinose:melezitose mixture. The frequency spectrum of the plot 144 illustrates that whereas the raffinose and melezitose are indistinguishable at the fundamental frequency, they are partially resolved at the third overtone, 146 ₁ and 146 ₂, and are fully resolved at the fifth overtone, 148 ₁ and 148 ₂. The harmonic/overtone analysis described in this disclosure, e.g., the pulse width duration sweeping process 100 illustrated in FIG. 7, may therefore be used to accurately identify a sample component of interest, and/or to distinguish a sample component of interest from another sample component.

The raffinose and melezitose mixture can alternatively be resolved at or near their fundamental frequencies if allowed to drift along a sufficiently long drift distance. This may be accomplished, for example, by employing the ion mobility spectrometer operating technique illustrated in FIG. 5 in an ion mobility spectrometer having a circular or so-called cyclotron geometry, as illustrated and described in co-pending PCT Publication No. WO 2008/028159 A2, filed Aug. 1, 2007, the disclosure of which has been incorporated herein by reference. Referring now to FIG. 12A, plots 150, 152 and 154 are shown of such an experiment in which raffinose and melezitose were separated in a circular or cyclotron geometry ion mobility spectrometer 160 having a cyclotron portion constructed of four conventional ion funnels, F2-F5 joined at each end by curved drift tube segments D1-D4, as illustrated in FIG. 12 B. Two of the curved drift tube segments D1 and D4 have Y-shaped geometries. The segment D1 selectively direct ions into the cyclotron portion via an ion source, e.g., an electrospray ion source, ESI, via a funnel/gate arrangement, F1/G1, and the segment D4 selectively extracts ions from the cyclotron portion and directs them through a cascaded funnel, F6, and drift tube segment, D5 to an ion detector, DET. Voltage sources and control circuitry, as illustrated and described above, are omitted from FIG. 12B for brevity.

Ions are introduced into the cyclotron via control of the gate, G1, and drift around the cyclotron via pulses applied to the various cyclotron sections which create electric fields in the cyclotron sections at a desired frequency or pulse rate as described hereinabove with respect to FIG. 5. The ions can be so directed around the cyclotron portion any number of times, or can be extracted from the cyclotron portion, via control of the illustrated ion gates G2 a and G2 b. The electric fields in F6 and D5 are illustratively constant.

Referring again to FIG. 12 a, the plot 150 represents the raffinose, R, and melezitose, M, ion peaks after 1¾ cycles of ion travel through the cyclotron portion of the instrument 160. The plot 152 similarly represents the raffinose, R, and melezitose, M, ion peaks after 3¾ cycles, and the plot 154 represents the raffinose, R, and melezitose, M, ion peaks after 6¾ cycles. While 1¾ cycles is sufficient to isolate each of the raffinose and melezitose ions, it is evident from FIG. 12 a that the two ion peaks separate further as the number of cycles increases.

Alternatively still, mixtures can be resolved in a linear drift tube, e.g., of the type illustrated in FIG. 1, at or near their fundamental frequencies by controlling the electric field activation sources in a manner that directs ions back and forth between the ends of the drift tube. More specifically, ions entering the ion inlet of the drift tube 14 are directed by the electric field activation sources toward the ion outlet of the drift tube 14 by selectively controlling the activation times and pulse widths of the electric field activation sources as described hereinabove. In this embodiment, prior to reaching the ion outlet, the control circuit 18 would control the electric field activation sources to reverse the direction of the sequentially applied electric fields to the cascaded drift tube segments such that the ions move linearly toward the ion inlet of the drift tube 14. As before, the duration of the pulse widths, PW, would determine the range of ion mobilities of the ions traversing the drift tube 14. In any case, this may be repeated any number of times to allow the ions to drift any desired distance. After drifting the desire distance, a gate at the ion outlet of the drift tube 14 would be activated to allow the ions to exit the ion outlet of the drift tube 14.

Alternatively or additionally in the linear or cyclotron configuration of the instrument, a non-destructive detector, e.g., rather than an ion counting detector 16, could be used that is configured to measure the image charge many times at a specified position within drift tube. Ion distributions could then be recorded as the frequency that ions pass the non-destructive detector. A conventional frequency transformation, e.g., Fourier transform, could then be used to back-calculate ion mobility.

Referring now to FIG. 13, a block diagram of one illustrative embodiment of a cascaded ion mobility spectrometer instrument 170 is shown that employs some of the concepts illustrated and described hereinabove with respect to FIGS. 1-12B. In the illustrated embodiment, the instrument 170 includes an ion source 12 having an ion outlet coupled to an ion inlet of a first ion mobility spectrometer (IMS1). An ion outlet of IMS1 is coupled to an ion inlet of a second ion mobility spectrometer (IMS2) having an ion outlet that is coupled to an ion detector 16. The IMS1 has an axial length of L1 and the IMS2 has an axial length of L2. A control circuit 180 includes a memory unit 182, and is electrically connected to a control input of an ion source voltage supply, V_(IS), having an output that is electrically connected to the ion source 12. The instrument 170 further includes a plurality of electric field activation sources, V₁-V_(Q), that are electrically connected between the control circuit 180 and IMS1 and IMS2, where Q may be any integer greater than 1. Illustratively, a subset of the electric field activation sources, V₁-V_(P), are dedicated to IMS1, and another subset, V_(P+1)-V_(Q) are dedicated to IMS2. Alternatively, V_(P+1)-V_(Q) may be omitted, and V₁-V_(P) may be used for both of IMS1 and IMS2. In any case, each of the foregoing components may be as described hereinabove with respect to the embodiment of FIG. 1. In one illustrative embodiment, the instrument 170 is operable just as described hereinabove with respect to any of FIGS. 1-12B, except that the ions are resolved over an effective drift tube length of L1+L2 rather than over the length of a single drift tube, such that IMS1 and IMS2 together form a single ion mobility spectrometer.

The ion mobility spectrometer instrument 170 of FIG. 13 may further include at least one additional voltage source, VF, which may be controlled by the control circuit 180 to produce one or more voltages that control one or more ion fragmentation units or other conventional device for inducing structural changes in ions within IMS1, IMS2 and/or positioned between IMS1 and IMS2. In embodiments in which the drift tubes of IMS1 and IMS2 are constructed according to the teachings of co-pending U.S. Patent Application Pub. No. US 2007/0114382, for example, an ion activation region of the type described therein may be positioned at the end of any one or more ion funnels that form IMS1 and/or IMS2. Alternatively, IMS1 and/or IMS2 may be modified in other embodiments to include one or more conventional structural change inducing devices or stages, e.g., one or more ion fragmentation stages, ion conformational change stages, and/or other conventional structural change inducing devices or stages, therein or interposed between IMS1 and IMS2. For example, such a structural change inducing device may be positioned between IMS1 and IMS2, and the ion mobility spectrometer 170 may be operated as described hereinabove to conduct fundamental frequency and/or overtone frequency analysis with IMS1, to then induce structural changes in ions emerging from IMS1, and to then conduct fundamental frequency and/or overtone frequency analysis with IMS2 on the ions in which structural changes were induced. Alternatively or additionally, the fragmented ions may be mobility filtered in a conventional manner prior to entering IMS2. Alternatively or additionally still, such fragmented and mobility-selected ions may be further fragmented and possibly further mobility selected any number of times prior to entrance into IMS2.

IMS1 may further include an operating condition selection unit 190, and IMS2 may likewise include an operating condition selection unit 200. The operating condition selection units 190 and 200 may be manually controlled via respective manual controls 194 and 204, and/or may be automatically controlled by the control circuit 180 via suitable electrical control lines 192 and 202 respectively. The operating condition selection units 190 and 200 are block diagram components that may represent conventional structures that control any one or more of the operating temperature of IMS1 and/or IMS2, the operating pressure of IMS1 and/or IMS2, the chemical make up and/or flow rate of gas, e.g., buffer gas, supplied to the ion pathway of IMS1 and/or IMS2, and the like. In the operation of the ion mobility spectrometer instrument 170, such as described hereinabove, IMS1 and IMS2 may be operated as described hereinabove and further with any one or more of, or with any combination of, the same or different drift tube lengths, L1 and L2, the same or different electrical field strengths applied by the electric field activation sources V₁-V_(Q), the same or different pulse shapes applied by the electric field activation sources V₁-V_(Q), the same or different pulse width durations, PW, applied by the electric field activation sources V₁-V_(Q), the same or different operating temperatures (e.g., T1 for IMS1 and T2 for IMS2), the same or different operating pressures, (e.g., P1 for IMS1 and P2 for IMS2), with the ions passing through IMS1 and IMS2 exposed to the same or different gasses (Gas1 for IMS1 and Gas2 for IMS2), or with ion fragmentation occurring within or between IMS1 and IMS2. Further details relating to some of these operational scenarios or modes are provided in U.S. Pat. No. 7,077,904, the disclosure of which is incorporated herein by reference.

It will be understood that the ion mobility spectrometer instrument illustrated in FIG. 13 and described herein represents only an example multiple drift tube instrument, and that the instrument may alternatively include any number of IMS units or drift tubes. Alternatively still, the IMS drift tubes in such an arrangement need not be linear, and the instrument 170 illustrated in FIG. 13 may include any number of non-linear IMS drift tube, such as two or more circular drift tubes of the type described in co-pending PCT Publication No. WO 2008/028159 A2, filed Aug. 1, 2007, the disclosure of which has been incorporated herein by reference.

While the invention has been illustrated and described in detail in the foregoing drawings and description, the same is to be considered as illustrative and not restrictive in character, it being understood that only illustrative embodiments thereof have been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected. 

1. An ion mobility spectrometer instrument comprising: a drift tube partitioned into a plurality of cascaded drift tube segments each defining an ion inlet at one end and an ion outlet at an opposite end, the plurality of cascaded drift tube segments further defining an ion elimination region between an ion outlet of each drift tube segment and an ion inlet of an adjacent drift tube segment, an ion source coupled to the ion inlet of a first one of the plurality of cascaded drift tube segments, a number, M, of electric field activation sources each operatively connected to one or more of the plurality of drift tube segments such that, when activated, each establishes a repulsive electric field in a different one of the first M ion elimination regions and in every following Mth ion elimination region, and also establishes an electric drift field in all remaining ion elimination regions and in all of the plurality of cascaded drift tube segments, and a control circuit configured to sequentially activate each of the number, M, of electric field activation sources for a time duration while deactivating the remaining number, M, of electric field activation sources a number of times to thereby cause only ions generated by the ion source that have a predefined ion mobility or range of ion mobilities to traverse the drift tube.
 2. The ion mobility spectrometer instrument of claim 1 wherein the ion inlet of the first one of the plurality of cascaded drift tube segments defines an ion inlet of the drift tube and the ion outlet of the last one of the plurality of cascaded drift tube segments defines an ion outlet of the drift tube, and wherein ions travel from the ion inlet of the drift tube through the ion outlet of the drift tube under the influence of the electric drift fields established by the number, M, of electric field activation sources, and wherein ions that do not have the predefined mobility or range of ion mobilities are filtered away by the repulsive electric fields established by the number, M, of electric field activation sources.
 3. The ion mobility spectrometer instrument of claim 1 wherein the ion source is configured to continuously generate ions, and wherein the continuously generated ions enter the ion inlet of the drift tube under the influence of the electric drift field sequentially established by each of the plurality, M, of electric field activation sources in the first drift tube segment.
 4. The ion mobility spectrometer instrument of claim 1 wherein the ion inlet of the first drift tube segment comprises an ion gate, and wherein the control circuit is configured to control the ion gate to selectively allow entrance of discrete packets of ions generated by the ion source into the ion inlet of the drift tube.
 5. The ion mobility spectrometer instrument of claim 1 wherein the ion source comprises at least one ion separation instrument configured to separate ions in time as a function of one or more molecular characteristics.
 6. The ion mobility spectrometer instrument of claim 5 wherein the at least one ion separation instrument includes at least one of a liquid chromatograph, a gas chromatograph, an ion mobility spectrometer, a mass spectrometer, and a capillary electrophoresis instrument.
 7. The ion mobility spectrometer instrument of claim 1 wherein each of the number, M, of electric field activation sources is programmable to establish, when triggered, the electric drift and repulsive fields for the time duration, and wherein the control circuit is configured to sequentially activate the number, M, of electric field activation sources for the time duration by sequentially triggering the number, M, of electric field activation sources.
 8. The ion mobility spectrometer instrument of claim 1 wherein each of the number, M, of electric field activation sources is programmable to establish, when triggered, the electric drift and repulsive fields for the time duration, and wherein the control circuit is configured to sequentially activate the number, M, of electric field activation sources for the time duration by triggering one of the number, M, of electric field activations sources with remaining ones of the number, M, of electric field activation sources being triggered by operation of a previously triggered one of the number, M, of electric field activation sources.
 9. The ion mobility spectrometer instrument of claim 1 wherein the time duration defines the mobility or range of mobilities of ions that will traverse the drift tube.
 10. The ion mobility spectrometer instrument of claim 1 wherein each of the number, M, of electric field activation sources is configured, when activated, to establish the electric drift and repulsive fields in the form of constant electric fields for the time duration.
 11. The ion mobility spectrometer instrument of claim 1 wherein each of the number, M, of electric field activation sources is configured, when activated, to establish the electric drift and repulsive fields in the form of linearly varying electric fields for the time duration.
 12. The ion mobility spectrometer instrument of claim 1 wherein each of the number, M, of electric field activation sources is configured, when activated, to establish the electric drift and repulsive fields in the form of non-linearly varying electric fields for the time duration.
 13. The ion mobility spectrometer instrument of claim 1 wherein the control circuit is configured to sequentially activate each of the number, M, of electric field activation sources for a first time duration while deactivating the remaining number, M, of electric field activation sources a number of times for a plurality of different time durations ranging between second and third time durations to thereby produce at the ion outlet of the drift tube ions at a number of overtone frequencies that are functionally related to the first time duration.
 14. The ion mobility spectrometer instrument of claim 13 further comprising an ion detector configured to detect ions exiting the ion outlet of the first drift tube and produce corresponding ion detection signals, wherein the control circuit is configured to convert the ion detection signals to the frequency domain for identification of ion intensity signals at the number of overtone frequencies and at a fundamental frequency defined by the first time duration.
 15. An ion mobility spectrometer comprising: a first drift tube partitioned into a plurality of cascaded drift tube segments each defining an ion inlet at one end and an ion outlet at an opposite end, the plurality of cascaded drift tube segments of the first drift tube further defining an ion elimination region between an ion outlet of each drift tube segment and an ion inlet of an adjacent drift tube segment, an ion source coupled to the ion inlet of a first one of the plurality of cascaded drift tube segments of the first drift tube, a first number, P, of electric field activation sources each operatively connected to one or more of the plurality of drift tube segments such that, when activated, each establishes a repulsive electric field in a different one of the first P ion elimination regions and in every following Pth ion elimination region, and also establishes an electric drift field in all remaining ion elimination regions and in all of the plurality of cascaded drift tube segments of the first drift tube, a structural change inducing device configured to induce structural changes in ions exiting the ion outlet of a last one of the plurality of drift tube segments of the first drift tube, a second drift tube partitioned into a plurality of cascaded drift tube segments each defining an ion inlet at one end and an ion outlet at an opposite end, the plurality of cascaded drift tube segments of the second drift tube further defining an ion elimination region between an ion outlet of each drift tube segment and an ion inlet of an adjacent drift tube segment, a second number, Q, of electric field activation sources each operatively connected to one or more of the plurality of drift tube segments of the second drift tube such that, when activated, each establishes a repulsive electric field in a different one of the first Q ion elimination regions and in every following Qth ion elimination region, and also establishes an electric drift field in all remaining ion elimination regions and in all of the plurality of cascaded drift tube segments of the second drift tube, and a control circuit configured to sequentially activate each of the number, P, of electric field activation sources for a first time duration while deactivating the remaining number, P, of electric field activation sources a first number of times to thereby cause only ions generated by the ion source that have a first predefined ion mobility or range of ion mobilities to traverse the first drift tube, the control circuit further configured to sequentially activate each of the number, Q, of electric field activation sources for a second time duration while deactivating the remaining number, Q, of electric field activation sources a second number of times to thereby cause only ions exiting the structural change inducing device that have a second predefined ion mobility or range of ion mobilities to traverse the second drift tube.
 16. The ion mobility spectrometer instrument of claim 15 wherein the ion source is configured to continuously generate ions, and wherein the continuously generated ions enter the ion inlet of the first drift tube under the influence of the electric drift field sequentially established by each of the plurality, P, of electric field activation sources in the first drift tube segment of the first drift tube.
 17. The ion mobility spectrometer instrument of claim 15 wherein the ion inlet of the first drift tube segment of the first drift tube comprises an ion gate, and wherein the control circuit is configured to control the ion gate to selectively allow entrance of discrete packets of ions generated by the ion source into the ion inlet of the first drift tube.
 18. The ion mobility spectrometer instrument of claim 1 wherein the ion source comprises at least one ion separation instrument configured to separate ions in time as a function of one or more molecular characteristics.
 19. The ion mobility spectrometer instrument of claim 18 wherein the at least one ion separation instrument includes at least one of a liquid chromatograph, a gas chromatograph, an ion mobility spectrometer, a mass spectrometer, and a capillary electrophoresis instrument.
 20. A method of separating ions as a function of ion mobility in a drift tube partitioned into a plurality of cascaded drift tube segments each defining an ion inlet at one end and an ion outlet at an opposite end, wherein the plurality of cascaded drift tube segments define an ion elimination region between an ion outlet of each drift tube segment and an ion inlet of an adjacent drift tube segment, the method comprising: deactivating a second electric repulsive field and a second electric drift field and establishing a first electric repulsive field in odd-numbered ones of the ion elimination regions while also establishing a first electric drift field in even-numbered ones of the ion elimination regions and in each of the drift tube segments for a time duration, deactivating the first electric repulsive field and the first electric drift field and establishing the second electric repulsive field in even-numbered ones of the ion elimination regions while also establishing a second electric drift field in odd-numbered ones of the ion elimination regions and in each of the drift tube segments for the time duration, and sequentially repeating the deactivating steps a number of times to thereby cause only ions entering the ion inlet of a first one of the plurality of cascaded drift tube regions that have a predefined ion mobility or range of ion mobilities to traverse the drift tube. 